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Ultrasound

  1. 1. ULTRASOUND Submitted to, Submitted by, Dr. MILAN ANAND IRAM ANWAR
  2. 2. ULTRASOUND • Therapeutic ultrasound is defined by any ultrasonic procedure that utilizes ultrasound for therapeutic applications. These procedures can include lithotripsy, cancer therapy, ultrasound hemostasis, HIFU, transdermal ultrasound drug delivery, targeted ultrasound drug delivery and ultrasound assisted thrombolysis. Ultrasound therapies include unfocused ultrasound and focused ultrasound (FUS), with the difference being the rate at which the sound waves penetrate the tissues. These high-frequency sound waves that measure between 800,000 Hz and 2,000,000 Hz stimulate the tissues beneath the skin’s surface via an applicator or transducer that stays in constantly moving direct contact with the patient’s skin.
  3. 3. Therapeutic ultrasound offers two types of effects, • Thermal • Non-thermal/mechanical 1.THERMAL EFFECT Thermal effects are characterized by the various absorption of the sound waves, while non-thermal and mechanical effects come from acoustic streaming, microstreaming and cavitation. Using a more continuous transmission of sound waves, thermal ultrasound increases heat and friction in relation to the microscopic vibrations it makes in the deep tissue molecules. This warming effect enhances healing and repair in the soft tissues through the increase of metabolism at the cellular level.
  4. 4. 2.NON –THERMAL EFFECT The non-thermal effects of US are now attributed primarily to a combination of CAVITATION and ACOUSTIC STREAMING There appears to be little by way of convincing evidence to support the notion of MICROMASSAGE though it does sound rather appealing. CAVITATION in its simplest sense relates to the formation of gas filled voids within the tissues & body fluids. There are 2 types of cavitation - STABLE & UNSTABLE which have very different effects. STABLE CAVITATION does seem to occur at therapeutic doses of US. This is the formation & growth of gas bubbles by accumulation of dissolved gas in the medium. They take apx. 1000 cycles to reach their maximum size. The `cavity' acts to enhance the acoustic streaming phenomena (see below) & as such would appear to be beneficial.
  5. 5. UNSTABLE (TRANSIENT) CAVITATION is the formation of bubbles at the low pressure part of the US cycle. These bubbles then collapse very quickly releasing a large amount of energy which is detrimental to tissue viability. There is no evidence at present to suggest that this phenomenon occurs at therapeutic levels if a good technique is used. There are applications of US that deliberately employ the unstable cavitation effect (High Intensity Focussed Ultrasound or HIFU) but it is beyond the remit of this summary. ACOUSTIC STREAMING is described as a small scale eddying of fluids near a vibrating structure such as cell membranes & the surface of stable cavitation gas bubble This phenomenon is known to affect diffusion rates & membrane permeability. Sodium ion permeability is altered resulting in changes in the cell membrane potential. Calcium ion transport is modified which in turn leads to an alteration in the enzyme control mechanisms of various metabolic processes, especially concerning protein synthesis & cellular secretions. The result of the combined effects of stable cavitation and acoustic streaming is that the cell membrane becomes ‘excited’ (up regulates), thus increasing the activity levels of the whole cell. The US energy acts as a trigger for this process, but it is the increased cellular activity which is in effect responsible for the therapeutic benefits of the modality
  6. 6. ULTRASOUND ENERGY Ultrasound (US) is a form of MECHANICAL energy, not electrical energy and therefore strictly speaking, not really electrotherapy at all but does fall into the Electro Physical Agents grouping. Mechanical vibration at increasing frequencies is known as sound energy. The normal human sound range is from 16Hz to something approaching 15-20,000 Hz (in children and young adults). Beyond this upper limit, the mechanical vibration is known as ULTRASOUND. The frequencies used in therapy are typically between 1.0 and 3.0 MHz (1MHz = 1 million cycles per second). Sound waves are LONGITUDINAL waves consisting of areas of COMPRESSION and RAREFACTION. Particles of a material, when exposed to a sound wave will oscillate about a fixed point rather than move with the wave itself. As the energy within the sound wave is passed to the material, it will cause oscillation of the particles of that material.
  7. 7. Clearly any increase in the molecular vibration in the tissue can result in heat generation, and ultrasound can be used to produce thermal changes in the tissues, though current usage in therapy does not focus on this phenomenon In addition to thermal changes, the vibration of the tissues appears to have effects which are generally considered to be non thermal in nature, though, as with other modalities (e.g. Pulsed Shortwave) there must be a thermal component however small. As the US wave passes through a material (the tissues), the energy levels within the wave will diminish as energy is transferred to the material. The energy absorption and attenuation characteristics of US waves have been documented for different tissues
  8. 8. ULTRASOUND WAVES : FREQUENCY - the number of times a particle experiences a complete compression/rarefaction cycle in 1 second. Typically 1 or 3 MHz . WAVELENGTH -the distance between two equivalent points on the waveform in the particular medium. In an ‘average tissue’ the wavelength at 1MHz would be 1.5mm and at 3 MHz would be 0.5 mm. VELOCITY - the velocity at which the wave (disturbance) travels through the medium. In a saline solution, the velocity of US is approximately 1500 m sec-1 compared with approximately 350 m sec-1 in air (sound waves can travel more rapidly in a more dense medium). The velocity of US in most tissues is thought to be similar to that in saline.
  9. 9. These three factors are related, but are not constant for all types of tissue. Average figures are most commonly used to represent the passage of US in the tissues. Typical US frequencies from therapeutic equipment are 1 and 3 MHz though some machines produce additional frequencies (e.g. 0.75 and 1.5 MHz) and the ‘Longwave’ ultrasound devices operate at several 10’s of kHz (typically 40-50,000Hz – a much lower frequency than ‘traditional US’ but still beyond human hearing range. ULTRASOUND BEAM, NEAR FIELD, FAR FIELD AND BEAM NON UNIFORMITY The US beam is not uniform and changes in its nature with distance from the transducer. The US beam nearest the treatment head is called the NEAR field, the INTERFERENCE field or the Frenzel zone. The behaviour of the US in this field is far from regular, with areas of significant interference. The US energy in parts of this field can be many times greater than the output set on the machine (possibly as much as 12 to 15 times greater). The size (length) of the near field can be calculated using r2/lambda where r= the radius of the transducer crystal and lambda = the US wavelength according to the frequency being used (0.5mm for 3MHz and 1.5mm for 1.0 MHz).
  10. 10. As an example, a 'crystal' with a diameter of 25mm operating at 1 MHz will have a near field/far field boundary at : Boundary = 12.5mm2/1.5mm 10cm thus the near field (with greatest interference) extends for approximately 10 cm from the treatment head when using a large treatment head and 1 MHz US. When using higher frequency US, the boundary distance is even greater. Beyond this boundary lies the Far Field or the Fraunhofer zone. The US beam in this field is more uniform and gently divergent. The ‘hot spots’ noted in the near field are not significant. For the purposes of therapeutic applications, the far field is effectively out of reach. One quality indicator for US applicators (transducers) is a value attributed to the Beam Nonuniformity Ratio (BNR). This gives an indication of this near field interference. It describes numerically the ratio of the intensity peaks to the mean intensity. For most applicators, the BNR would be approximately 4 - 6 (i.e. that the peak Example of an Ultrasound Beam Plot
  11. 11. intensity will be 4 or 6 times greater than the mean intensity). It is considered inappropriate to use a device with a BNR value of 8.0 or more. Because of the nature of US, the theoretical best value for the BNR is thought to be around 4.0 though some manufacturers claim to have overcome this limit and effectively reduced the BNR of their generators to 1.0. ULTRASOUND BEAM, NEAR FIELD, FAR FIELD AND BEAM NON UNIFORMITY • The US beam is not uniform and changes in its nature with distance from the transducer. The US beam nearest the treatment head is called the NEAR field, the INTERFERENCE field or the Frenzel zone. • The behaviour of the US in this field is far from regular, with areas of significant interference. The US energy in parts of this field can be many times greater than the output set on the machine (possibly as much as 12 to 15 times greater). The size (length) of the near field can be calculated using r 2 /lambda where r= the radius of the transducer crystal and lambda = the US wavelength according to the frequency being used (0.5mm for 3MHz and 1.5mm for 1.0 MHz).
  12. 12. • As an example, a 'crystal' with a diameter of 25mm operating at 1 MHz will have a near field/far field boundary at : Boundary = 12.5mm2 /1.5mm 10cm thus the near field (with greatest interference) extends for approximately 10 cm from the treatment head when using a large treatment head and 1 MHz US. • When using higher frequency US, the boundary distance is even greater. Beyond this boundary lies the Far Field or the Fraunhofer zone. • The US beam in this field is more uniform and gently divergent. The ‘hot spots’ noted in the near field are not significant. For the purposes of therapeutic applications, the far field is effectively out of reach. • One quality indicator for US applicators (transducers) is a value attributed to the Beam Nonuniformity Ratio (BNR). This gives an indication of this near field interference. It describes numerically the ratio of the intensity peaks to the mean intensity. For most applicators, the BNR would be approximately 4 - 6 . intensity will be 4 or 6 times greater than the mean intensity).
  13. 13. ULTRASOUND TRANSMISSION THROUGH THE TISSUES • All materials (tissues) will present an impedance to the passage of sound waves. The specific impedance of a tissue will be determined by its density and elasticity. • In order for the maximal transmission of energy from one medium to another, the impedance of the two media needs to be as similar as possible. • Clearly in the case of US passing from the generator to the tissues and then through the different tissue types, this can not actually be achieved. • The greater the difference in impedance at a boundary, the greater the reflection that will occur, and therefore, the smaller the amount of energy that will be transferred. • It is considered inappropriate to use a device with a BNR value of 8.0 or more. Because of the nature of US, the theoretical best value for the BNR is thought to be around 4.0 though some manufacturers claim to have overcome this limit and effectively reduced the BNR of their generators to 1.0.
  14. 14. The difference in impedance is greatest for the steel/air interface which is the first one that the US has to overcome in order to reach the tissues. To minimise this difference, a suitable coupling medium has to be utilised. If even a small air gap exists between the transducer and the skin the proportion of US that will be reflected approaches 99.998% which means that there will be no effective transmission. The coupling media used in this context include water, various oils, creams and gels Ideally, the coupling medium should be fluid so as to fill all available spaces, relatively viscous so that it stays in place, have an impedance appropriate to the media it connects, and should allow transmission of US with minimal absorption, attenuation or disturbance. The addition of active agents (e.g. anti-inflammatory drugs) to the gel is widely practiced, but remains incompletely researched.
  15. 15. Ultrasound Application - The Critical Angle In addition to the reflection that occurs at a boundary due to differences in impedance, there will also be some refraction if the wave does not strike the boundary surface at 90. Essentially, the direction of the US beam through the second medium will not be the same as its path through the original medium - its pathway is angled. The critical angle for US at the skin interface appears to be about 15. If the treatment head is at an angle of 15 or more to the plane of the skin surface, the majority of the US beam will travel through the dermal tissues (i.e. parallel to the skin surface) rather than penetrate the tissues as would be expected.
  16. 16. The physiological effects of ultrasound are almost identical to those of Pulsed Shortwave and Laser therapy PULSED ULTRASOUND Most machines offer the facility for pulsed US output, and for many clinicians, this is a preferable mode of treatment. Until recently, the pulse duration (the time during which the machine is on) was almost exclusively 2ms (2 thousandths of a second) with a variable off period. Some machines now offer a variable on time though whether this is of clinical significance has yet to be determined. Typical pulse ratios are 1:1 and 1:4 though others are available (see dose calculations). In 1:1 mode, the machine offers an output for 2ms followed by 2ms rest. In 1:4 mode, the 2ms output is followed by an 8ms rest period. The effects of pulsed US are well documented and this type of output is preferable especially in the treatment of the more acute lesions.
  17. 17. THERAPEUTIC ULTRASOUND : CONTRAINDICATIONS AND PRECAUTIONS CONTRAINDICATIONS : • Do not expose either the embryo or foetus to therapeutic levels of ultrasound by treating over the uterus during pregnancy • Malignancy (history of malignancy is NOT a contraindication – DO NOT treat over tissue that is, or considered to be malignant) • Tissues in which bleeding is occurring or could reasonably be expected (usually within 4-6 hours of injury but may be longer in some instances and for some patients) • Significant vascular abnormalities including deep vein thrombosis, emboli and severe arteriosclerosis / atherosclerosis (if increase in local blood flow demanded by the treatment can not reasonably be delivered) • Patients with Haemophilia not covered by factor replacement Application over : o The eye o The stellate ganglion o The cardiac area in advanced heart disease & where pacemakers in situ o The gonads o Active epiphyses in children
  18. 18. PRECAUTIONS : • Always use the lowest intensity which produces a therapeutic response • Ensure that the applicator is moved throughout the treatment (speed and direction not an issue) • [not necessary with LIPUS applications or the newly advocated STATUS application] • Ensure that the patient is aware of the nature of the treatment and its expected outcome • If a thermal dose is intended, ensure that any contraindications that apply have been considered • Caution is advised in the vicinity of a cardiac pacemaker or other implanted electronic device • Continuous ultrasound is considered unwise over metal implants HAZARDS : Reversible blood cell stasis can occur in small blood vessels if a standing wave is produced while treating over a reflector such as an air/soft tissue interface, soft tissue/bone or soft tissue/metal interface whilst using a stationary applicator. This having been said, I can identify no evidence that this occurs at 'normal' therapeutic levels and with a moving head application method. Treatment with a stationary treatment head is considered bad practice in the normal therapy environment (LIPUS excepted).
  19. 19. Phonophoresis : It is the use of ultrasound to penetrate topical medicine deeper below the skin than by applying it on its own. To be used with pharmacological agents such as anti-inflammatory steroids and local anesthetics. The sound waves from the ultrasound carry the medication under the skin to the muscle or tissue to more effectively absorb the medicine. Drugs requiring specific dosage should not be administered by phonophoresis because it is difficult to be controlled accurately. Steps of Use: 1. Apply drug directly to clean skin 2. Apply ultrasound conductive gel over the drug on the skin 3. Ultrasound is turned on and wand is placed over gel/drug content 4. Wand is moved in a circular motion over an area no larger than three (3) times the size of the wand head 5. This should be done for four (4) to six (6) minutes based on the size of treatment area 6. There should be a warming sensation caused by the use of ultrasound
  20. 20. Indications: Localized inflammation of a tendon Localized inflammation of a bursa Localized inflammation of a joint Contraindications Do not use over Genitals Stomach of a pregnant woman Epiphyseal plates Eyes Open wounds Pacemakers Breast implants

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